In the operation of any type of plant using pulverized coal for fuel, it is common practice to supply raw coal to a ball mill pulverizer to reduce the size of the coal and mix it with air. The main component of the typical ball mill is a large drum with hollow trunnions at each end forming an extension of the drum and providing space for introduction of the raw coal to the mill. The trunnions also define a bearing surface for permitting rotation of the drum. The drum is filled with a charge of forged steel balls to a level just below the bottom of the trunnions. As the drum is rotated, the balls are continuously lifted and tumbled by gravity onto the coal to pulverize the coal. Classifiers are usually located at each end of the mill and serve as separators, so that as the pulverized coal is passed from the grinding zone of the mill to the pulverizer, and oversized particles are rejected by gravity separation in a low velocity central section of the classifier and by centrifugal separation. The rejected material is mixed with the incoming raw coal feed and returned through the trunnion by screw conveyors for further grinding.
Rotation of the drum and trunnions generates frictional forces between the bearing surfaces of the trunnions and associated bearing assemblies which can significantly reduce the efficiency of operation of the system and lead to excessive wear of components. Many ball mill pulverizer assemblies are therefore equipped with means for continuously lubricating the bearing surfaces. A problem with assemblies of this type is that coal dust or other foreign material can infiltrate the bearing surfaces and surrounding areas thereby contaminating the bearing lubricant. The coal dust acts like an abrasive and accelerates wear of the bearing surfaces, and further adversely affects the lubricant pumping system usually associated with assemblies of this type.
A fabric such as felt or similar material can be used to seal the area surrounding the bearing surfaces. However, adequate contact of the fabric in sealing engagement with the trunnion is often difficult to maintain in view of minor displacements of the sealing surface caused during rotation by asymmetry and precision of the trunnion, as well as thermal expansion and contraction of the mill, etc. Fabric seals will also dry, wear and deteriorate over an extended period of operation, resulting in failure of the seal.